This invention relates to a position finding apparatus in an electric discharge machine and, more particularly, to improvements in a position finding apparatus power supply unit of a simplified circuit construction in which a pulse oscillator is protected.
When performing electric discharge machining, such as wire-cut or engraving electric discharge machining, it is necessary to determine the relative positions of a workpiece and tool. A position finding apparatus is provided for this purpose. Conventionally, a DC power supply is used as the power supply of the position finding apparatus. However, since this is a single-polarity power supply, phenomena occur such as flawing of the workpiece surface due to an electrolytic effect between the workpiece and tool, as well as the attraction of the tool to the workpiece, particularly in a case where the tool is a wire used in performing wire-cut electric discharge machining. The result is that a decline in position finding precision is inevitable. Accordingly, it has been proposed to use, as the position finding power supply, an arrangement in which voltage pulses of both positive and negative polarity are generated and impressed across the workpiece and tool, thereby making the average voltage zero in order to solve the aforementioned problem. FIG. 1 is an overall circuit diagram for describing such prior art.
In FIG. 1, numeral 1 denotes a power supply unit for position finding. The power supply unit is composed of a pulse oscillator 11, a protective diode 12 for assuring that a reverse voltage does not occur, and a ballast transformer 13. R.sub.1, R.sub.2 denote resistors, P a tool electrode and W a workpiece. Numeral 2 designates a position finding signal generating circuit comprising an absolute value circuit 21, a filter 22 and a comparator 23. Numeral 3 represents a relay and 4 a machining power supply unit equipped with a transistor-controlled capacitor discharge circuit (not shown). With this construction, pulses from the pulse oscillator 11 are applied to the ballast transformer 13 when the relay 3 turns on the position finding power supply, whereby bipolar pulses are applied across the tool electrode P and workpiece W, so that the voltage across them becomes zero when the two are in contact. This voltage is compared with a reference voltage Vr by a comparator 23 which produces an output signal indicating the short-circuited state.
The above-described conventional power supply for position finding in an electric discharge machine uses a ballast transformer in order to generate a pulse-shaped voltage having positive and negative polarities. Therefore, at the instant the workpiece and tool are short circuited in the course of performing position finding, zero potential is developed across the workpiece and tool, thus changing the voltage. The change in voltage causes a back electromotive force to be produced by the ballast transformer 13, and the pulse oscillator 11 is damaged by the back electromotive force. To avoid this problem, the protective diode is provided or a protective resistor is used, so that a reverse voltage will not be impressed upon the pulse oscillator 11, or a limit switch is provided to prevent the reverse voltage from being applied to the pulse oscillator 11. This is not a satisfactory countermeasure, however, as costs are raised and the pulse waveform develops distortion.